RTF is that silicone gasket stuff you buy at NAPA or any parts store. Just go in and ask for it, they'll show you what you need.

Yes, there's an allen head bolt on your front differential closest to your driver's side spring perch u-bolt. It's an 8mm allen head. At least mine was. I believe 5/16" will work.

I think the person at your dealership is confused. If you have a limited slip differential, it uses a slip disc clutch system to allow it to transfer energy from one wheel to another when you experience slip. If there's no friction modifier in the differential fluid, the discs will always be locked, thus you'll experience chirping around corners, etc. because the diff is locked.

A locking differential, ie Detroit locker, etc. are permanently locked (unless soft lockers) and they do not require slip additive. They don't have clutches in them. They're always locked. So I don't know why on earth you'd need a slip modifier in your differential fluid if you have a true locker. A soft locker unlocks itself around corners, but they make a clicking sound and they remain fully locked in the straight, last I knew.

This is my understanding of limited slip and locker diff configurations.

 

Okay, don't listen to that chowder head. Other guys have told me they're running syn 75w-140 in front & rear diffs now (even the Dana 80). There was a thread on it the other day, and I was saying I wouldn't change from what the manual says unless I saw a TSB. You DEFINITELY need friction modifier in LS diffs. No doubt. I put Royal Purple in mine, and it had the friction modifier in it already. It's working well. I also know a lot of guys are running 75w-140 in the front with no problems.

At this point, I'd say decide if you want to find the TSBs and go with 75w-140 or stick with what's in the manual. Just double-check the bottle to find out if you need the FM or not. IIRC, you DON'T need it in the front diff, just in the rear. Somebody check me...

 

Thanks for all the input guys: Here is the summary for all newbies to differentials.
I called another knowledgeable dealer and he answered all my questions.
#1
Dana 80/60 needs 75W-90 Synthetic.
#2
A big YES, all LS diffs require friction modifies. So, use aftermarket fluids that comes with friction modifier in them. Otherwise you need to measure the amount of friction modifier (FM) that you put in. Adding too much FM is a very bad idea. Why, read post number 4.
#3
If you are replacing the diff cover, yes, you need gasket and silicon sealant for a tighter seal. Mote info here How to Change a Differential Cover and Gasket on a Car | eHow.com
#4
To address quantity of oil, pour as much as you can until the drain plug overflows a few ounces.
#5
After this, drive around the block making tight turns and listen for any unusual noise from diff.

#6
If your truck is a 4x4, engage the front diff at low speeds so that the oil moves around.

 

Thanks for all your input Jason and Joe.

 

Never heard of RTF. I have heard of RTV though. Perhaps that's what you've been meaning?

 

Yessir, that is what I meant.

 

LOL... Okay -- just checking.

 

You're welcome

 

Well, I was able to finish up the new differential look over the weekend. Here is the list of all the problem/solution:
#1
Factory specs say 4.0 L for a DANA 80 rear, mine took a lot more. I would like to believe some where in the vicinity of 5L.
#2
Between the front and back I spent $152 (before tax) in differentials fluids. I went with Royal purple synthetic 75W-90 with FM in it. $18.99 a bottle.
#3
I had difficulty removing the front diff cover bolts, partly because the sealant has been sticking to the bolts for over 9 years and also because there is one allen type bolt in the front! It is pretty tight space to mess around, so I had to jack up to get more head room.
#4
My new diff covers had 18 size drain plug on the rear and front was weird, a 15 size female nut type!!!!. Now, I had no tool with me to unscrew it, so I devised my own: I took a 15 size bolt (one and half inch long) and bolted two 15 size nuts to it( you see where I’m going…). Used the bolt head to plug the drain plug head and 15 size spanner to unplug the drain plug using the mid nut to loosen!!!! Pretty smart haaayee?


==========END of STORY===========

 

Just a hint, leave the top bolt loosely in the hole when you drain the differential... Then you don't drop the diff cover in the dirt like I did while it was draining, then take the cover off and do the gasket. Oh, and the front is an open diff and doesn't need modifier. For what its worth, I don't run any modifier in my LS and have found it locks up much better...